In a corner of the Brooklyn Navy Yard, on a factory floor that resembles an oversized assembly line, workers are building entire apartments in days. Most New Yorkers might not realize it, but the tallest prefab building in the country—and maybe the world—is currently taking shape not far from where they live. Gizmodo recently got a chance to visit the space and watch it come together.

First of all, some backstory. The idea for "B2," as this tower is called, came about more than a decade ago, as the battle over Barclays Center—the Brooklyn Nets’ stadium in downtown Brooklyn—raged. Developer Forest City Ratner promoted the project by emphasizing the jobs, economic development, and affordable housing (2,250 units, to be exact) it would bring to the area. Its slogan was “Jobs, Housing, and Hoops.”

B2, on the right, in a rendering.

That was two years before the economic collapse. Since then, Forest City has struggled to build the other structures, citing the lagging economy for numerous delays and redesigns. A decade after the project began, nothing besides the arena had been built on the site.

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Then, last fall, Forest City unveiled a plan that it claimed would give the project the bump it needed to reach completion: The creation of a new company, FCS Modular, that would oversee the construction of the first tower of housing—which now, the developer explained, would be built using prefab modules.

The 32-story tower, called B2, would be the tallest prefab building in the world. And it would be assembled at a warehouse right here in Brooklyn.

The Brooklyn Navy Yard sits in a crook in the shoulder of downtown Brooklyn, right across the water from Lower Manhattan. When the Navy purchased it in 1801, it served as a vital node in the emerging military-industrial complex of early America: Its workers clad the very first iron-clad ship, and, during World War II, some 70,000 New Yorkers worked round-the-clock shifts to build boats and ammunition for American troops. The last ships were built in the 1970s, and the activity in the Navy Yard slowed—until the 2000s, when a group of businesses in rapidly-gentrifying Brooklyn moved in.

Today, the Navy Yard bustles with activity. It’s home to hundreds of small businesses and plenty of large manufacturing operations, too—as well as the largest production studio this side of Los Angeles.

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In a massive warehouse on the northern side of the Yard, FCS has set up one of the largest prefab factories on the planet: A perfectly-engineered assembly line where roughly 100 workers are putting together 930 modules that will eventually become B2.

One of the biggest hurdles with prefab isn’t just structural integrity or waterproofing, but the workers themselves: Prefab has long been perceived as a way for developers to avoid paying construction workers and, thus, to bypass unions altogether. So one of FCS’s biggest successes has actually been a legal one: The formation of a new, non-jurisdictional “Modular Division,” made up of union carpenters, iron workers, painters, plumbers, electricians, and more. It’s this agreement that makes it possible to use 100 percent union labor at the Navy Yard site, where small teams from each trade work on assembling each module simultaneously.

The modules begin their lives as simple steel boxes, mere skeletons of the apartments they’ll become in less than a week. Stacked outside the factory, each metal box is edged with extra beams that form Vierendeel trusses—the sort commonly found on bridges.

One of the main reasons prefab isn’t used to build tall buildings is because it’s hard to stabilize a stack of boxes against lateral forces, such as wind. Imagine a stack of Jenga blocks: To hold their form, the blocks need the plastic shell of packaging with which they arrived. Likewise, a stack of prefab modules need extra cross-bracing to stay put during storms and seismic activity.

This is a problem that the engineers at FCS (and Arup, the collaborating engineering firm) are quite proud of solving; when the workers install the modules later this year, they’ll also construct a framing lattice of extra steel cross-bracing inside the warren of boxes.

The frames are moved into the factory and, over the course of roughly seven days, workers from the Modular Division install drywall, windows, and even lighting fixtures in each apartment—which come in studio, one, two, and three-bedroom varieties.

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Everything is done according to a carefully-choreographed schedule: Many of the workers at the factory are simply in charge of making sure that the assembly line never runs out of the right plumbing fixture or electrical wires.

For example, at one end of the factory floor sit roughly ten metal platforms where the bathrooms are assembled. This is the most complex part of each apartment—it takes longer to assemble one bathroom than it does a whole module—because of the myriad plumbing and electrical fittings that go into it.

On one side, workers organize pieces of pipe into neatly-labeled burlap bags. These are put into rolling wooden shelves, which are wheeled out next to each bathroom like giant moving toolboxes. This way, the team of workers focusing on a particular bathroom have access to every fitting and pipe they’ll need to finish the job.

So, how are these modules—which, in essence, are finished homes—actually moved?

Workers use a massive vehicle that can lift 100,000 pounds.

When these room-sized LEGO blocks are finally trucked to the Atlantic Yards site, workers will bolt them together using an elaborate series of failsafe mechanisms. Astonishingly, no welding is involved, due to the fire hazard presented by all the finishings.

Outside the warehouse, four finished modules are being put together as a test. The vibrant red of the final product is visible under the peeling protective plastic strips.

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It’s an unlikely scenario. Prefabricated architecture has been a white whale in the design community for more than a century, steeped in utopian overtones. The fact that it’s being cauterized to build a megaproject like Atlantic Yards, is, well, a little ironic.

What will happen to this buzzing factory space when all 930 modules are complete? Roger Krulak, a Senior Vice President at Forest City Ratner who gave us a tour of the factory, explains that B2 is a case study—if all goes according to plan, it’ll serve as a model for other urban developments throughout the city. In theory, this factory could become the birthplace of dozens of other buildings over the next few decades, a kind of hive churning out new architectural forms for national distribution.

“Modular housing has developed incredibly well,” Krulak said as we left the factory floor. “And we believe it’s the same thing with larger buildings—probably more so—because of the economies of scale.” Krulak is realistic about the fact that prefab has been just over the horizon for decades. In fact, his own father was involved in a modular housing company called—in true 1960s style—Space Homes. “The technology hadn’t really caught up,” he adds. “And now it’s come of age.”